This invention relates to color purity adjustment of cathode ray tubes for color television receivers.
Color display systems such as utilized in color television receivers include a cathode ray tube in which three electron beams are modulated by color-representative video signals. The beams impinge on respective color phosphor areas on the inside of the tube viewing screen through apertures in a shadow mask. To accurately reproduce a color screen, the three beams must be substantially converged at the screen at all points on the raster. The deflection center of each of the three beams must be correctly located in the yoke deflection plane to establish color purity. Incorrectly located deflection centers, due to such factors as incorrect placement of the deflection yoke, tolerances in the manufacture of the electron beam guns, and their assembly into the cathode ray tube neck, frequently result in color misregistration.
Many color purity devices include structure for producing adjustable magnetic fields. The devices are placed over the neck of the cathode ray tube, and the magnetic fields are appropriately adjusted to provide for color purity of the electron beams. Such adjustment is accomplished by moving magnetic field producing elements, by rotating magnetized rings about the cathode ray tube neck, or by rotating cylindrical magnets about an axis.
Other color purity devices, such as disclosed in German Provisional Pat. No. 2,611,633, filed Mar. 19, 1976, published Oct. 21, 1976, by Piet Gerard Joseph Barten et al., produce permanent nonadjustable magnetic fields. In a first step, an auxiliary device having eight coils circumferentially located is placed around the cathode ray tube neck. Appropriately valued DC currents flowing through the coils establish a magnetic field which provide for color purity of the electron beams. The values of the DC currents provide data to a magnetizing apparatus which in a second step magnetizes regions within a sheath or strip of magnetic material producing the aforementioned permanent nonadjustable magnetic fields. The magnetized strip, when placed over the neck of the cathode ray tube, establishes the color purity of the electron beams.
It is desirable, when using such a magnetic strip, to eliminate the step of utilizing an auxiliary device for determining the locations within the strip where magnetized regions are to be established. A magnetizing apparatus, not utilizing such an auxiliary device, should have magnetizing areas arranged to facilitate uncomplicated operation when directly performing color purity operations.
For an in-line cathode ray tube with three in-line electron beams and a slot shadow mask with vertical slot apertures, color purity correction requires only horizontal, like-direction motion of all three beams. The magnetic field produced by the permanently magnetized regions need only have a vertical component perpendicular to the in-line axis of the cathode ray tube to produce the horizontal motion.
As color purity correction may require large motions, the magnetic strip must be capable of producing a sufficiently strong color purity magnetic field. Furthermore, the correction introduced by the color purity magnetic field must not introduce any substantial misconvergence of the electron beams, that is, the motion of all three electron beams should be in substantially identical directions and of substantially identical magnitudes.